The single biggest omission from Kingdom Hearts III was the lack of characters from the Final Fantasy series, which were once an integral part of the franchise. It turns out that there was a reason why the established Final Fantasy characters were absent from Kingdom Hearts III and it has to do with the amount of content that had to be covered in the story.
The biggest selling point of the original Kingdom Hearts was that it combined characters from both the Final Fantasy video game series and the animated movies and TV shows produced by Disney. The Final Fantasy characters only played a minor role in the story of the series, but their popularity left many fans wanting to see more of them. Kingdom Hearts III lacked representation from the Final Fantasy series, which was a point of contention for fans who had hoped to see a resolution to Cloud and Sephiroth's battle in Kingdom Hearts II.
The reason why there was a lack of Final Fantasy representation in Kingdom Hearts III was revealed when co-director Tai Yasue spoke to Game Informer at GDC. Yasue stated that the developers wanted to focus on resolving the storylines involving the original Kingdom Hearts characters and that he was surprised that the fans were so disappointed about the absence of the Final Fantasy characters.
"We had a lot of original characters. It's the conclusion of the Xehanort saga and we really wanted to concentrate on the main storyline of Kingdom Hearts, such as Sora and Roxas, for the example. They're having all these stories in other games and we wanted them to conclude."
The Kingdom Hearts series is notorious for its complex and nonsensical story, which meant that the developers of Kingdom Hearts III had a lot of ground to cover if they wanted to close the book on many of the plotlines that had been developing for over a decade. It's just a shame that the Final Fantasy characters had to be cut from the game in order to make room for other storylines, as they had helped to propel the series to success.
It wasn't just the Final Fantasy fans who were left disappointed by Kingdom Hearts III, as there was no representation from the MCU or Star Wars franchises, which was something that a lot of people were expecting. There was a lot of focus on Pixar franchises and Tetsuya Nomura has said that he wouldn't have made Kingdom Hearts III if he had not been allowed to use worlds based on the likes of Toy Story, so it's obvious that the Disney worlds were always going to be the top priority for the developers of Kingdom Hearts III.
All is not lost for the Kingdom Hearts III fans who want to see a resolution to the stories of the Final Fantasy characters, as paid DLC is coming to the game in the future which will take the place of the updated rereleases that the series has seen in the past. It's possible that the Final Fantasy characters are being saved for one last epic piece of story DLC that will wrap up their plotlines.
Source: Game Informer